1. Field of the Invention
The present invention relates to a spindle device of machine tool such as a machining center that performs various types of machining by mounting a tool at a point of a spindle and rotating the tool at a high speed.
2. Description of the Related Art
In a general spindle device in machine tool, a spindle is rotatably supported in a housing, a rod is movably supported along an axial direction in the spindle, a collet that can clamp the tool is mounted at the point of the rod, and the rod is moved against a biasing force of a biasing unit of a disc spring by a drive, while the rod is biased by a plurality of disc springs in a tool clamping direction by the collet, thereby enabling unclamping of the tool by the collet.
In the spindle device in machine tool configured in this manner, the collet clamps the tool by the biasing force of the disc spring. Therefore, a decrease in the biasing force of the disc spring largely affects machining accuracy by the tool. That is, if the biasing force of the disc spring decreases due to long time use, the collet cannot appropriately clamp the tool, and therefore not only a force of constraint of the spindle with respect to the tool decreases to decrease the machining accuracy by cutting of products, but also a surface damage such as fretting occurs in a clamping portion between the spindle and the tool.
Accordingly, for example, Patent documents 1 and 2 disclose a unit that monitors a biasing force of a spring for clamping a tool to a spindle in a spindle device in machine tool.
A tool-clamping-spring monitoring device in a spindle device of machine tool described in the Patent document 1, a pressure sensor measures a spring force of a disc spring at the time of moving a draw bar in a tool releasing direction by a hydraulic cylinder, a comparator compares the spring force measured by the pressure sensor with a predetermined reference value, and a determination unit determines a state of the disc spring based on a comparison result of the comparator. Further, in a spindle device of machine tool described in the Patent document 2, a disc-like locking dog is fitted to a rear end of a rod installed movably in the spindle, and a displacement sensor is installed on a cylinder positioned at a rear of the spindle to move the rod, which detects the position of the rod, to thereby determine whether the tool is normally clamped by a collet at a front end of the rod.    [Patent document 1] Japanese Patent Application Laid-Open No. 2003-220504    [Patent document 2] Japanese Patent Application Laid-Open No. H05-050359